Danfoss VLT AQUA Design Manual page 24

2. Introduction to VLT AQUA Drive
2.9.2. Closed Loop Control Relevant Parameters
The drive's Closed Loop Controller is capable of handling more complex applications, such as situations where a conversion function is applied to the
feedback signal or situations where multiple feedback signals and/or setpoint references are used. The below table summarizes the additional parameters
that may be useful in such applications.
2
Parameter
Feedback 2 Source
Feedback 3 Source
Feedback 1 Conversion
Feedback 2 Conversion
Feedback 3 Conversion
Reference Feedback
Feedback Function
Setpoint 1
Setpoint 2
Setpoint 3
Setpoint Adjustment Factor
PID Start Speed [RPM]
PID Start Speed [Hz]
On Reference Bandwidth
PID Anti Windup
PID Differentiation Time
PID Diff. Gain Limit
Flow Compensation
Square-linear Curve Approximation
Work Point Calculation
Speed at No-Flow [RPM]
Speed at No-Flow [Hz]
Speed at Design Point [RPM]
Speed at Design Point [Hz]
Pressure at No-Flow Speed
Pressure at Rated Speed
Flow at Design Point
Flow at Rated Speed
Lowpass Filter Time :
Analog Input 53
Analog Input 54
Digital (pulse) input 29
Digital (pulse) input 33
24
Par. No. Description of function
20-03 Select the source, if any, for Feedback 2 or 3. This is most commonly a drive
20-06 analog input, but other sources are also available. Par. 20-20 determines how
multiple feedback signals will be processed by the drive's Closed Loop Control-
ler. By default, these are set to
20-01 These are used to convert the feedback signal from one type to another, for
20-04 example from pressure to flow.
Flow = Pressure
20-07
20-12 For setting the unit used for setpoint reference and feedback.
20-20 When multiple feedbacks or setpoints are used, this determines how they will
be processed by the drive's Closed Loop Controller.
20-21 These setpoints can be used to provide a setpoint reference to the drive's
20-22 Closed Loop Controller. Par. 20-20 determines how multiple setpoint references
20-23 will be processed. Any other references that are activated in par. group 3-1*
20-29 will add to these values.
Par. 20-29 can be used to reduce the setpoint at low flow benefiting from a
reduced pipe resistance at reduced flow.
20-82 The parameter that is visible will depend on the setting of par. 0-02, Motor
20-83 Speed Unit. In some applications, after a start command it is important to
quickly ramp the motor up to some pre-determined speed before activating the
drive's Closed Loop Controller. This parameter defines that starting speed.
20-84 This determines how close the feedback must be to the setpoint reference for
the drive to indicate that the feedback is equal to the setpoint.
On
20-91 [1] effectively disables the Closed Loop Controller's integral function when
it is not possible to adjust the output frequency of the drive to correct the error.
This allows the controller to respond more quickly once it can again control the
Off
system. [0] disables this function, making the integral function stay active
continuously.
20-95 This controls the output of the drive's Closed Loop Controller based on the rate
of change of feedback. While this can provide fast controller response, such
response is seldom needed in Water systems. The default value for this pa-
rameter is Off, or 0.00 s.
20-96 Because the differentiator responds to the rate of change of the feedback, a
rapid change can cause a large, undesired change in the output of the control-
ler. This is used to limit the maximum effect of the differentiator. This is not
active when par. 20-95 is set to Off.
22-80 It is sometimes the case that it is not possible for a pressure transducer to be
22-81 placed at a remote point in the system and it can only be located close to the
22-82 fan/pump outlet. Flow compensation operates by adjusting the setpoint ac-
22-83 cording to the output frequency, which is almost proportional to flow, thus
22-84 compensating for higher losses at higher flow rates.
22-85 These parameters are used for setting up flow compensation.
22-86
22-87
22-88
22-89
22-90
6-16 This is used to filter out high frequency noise from the feedback signal. The
6-26 value entered here is the time constant for the low pass filter. The cut-off fre-
5-54 quency in Hz can be calculated as follows:
5-59
F cut
=
off
−
2π
Variations in the feedback signal whose frequency is below F
by the drive's Closed Loop Controller, while variations at a higher frequency are
considered to be noise and will be attenuated. Large values of Lowpass Filter
Time will provide more filtering, but may cause the controller to not respond to
actual variations in the feedback signal.
®
MG.20.N3.02 - VLT is a registered Danfoss trademark
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VLT AQUA Drive Design Guide
No function
[0].
1
T lowpass
will be used
cut-off